Many kinds of coil springs having linear spring properties have been used in many kinds of electric and electronic equipments or mechanical equipments. Further, coil springs having non-linear properties have been required instead of springs having linear properties in order to optimize the performance of the equipments.
In case of a brush type small DC motor widely used in the electrical and electronic equipments, for example, a relation between a pressure applied to a brush and an abrasion rate of the brush is shown in FIG. 1. As shown in FIG. 1, an electrical abrasion due to the commutation spark is increased when the brush pressure is reduced, and a mechanical abrasion is increased when the brush pressure is increased.
In case that a coil spring having a linear spring property is used in the brush type small DC motor, the brush pressure is in the range of the mechanical abrasion at an initial stage of the motor operation, and when the abrasion of the brush is increased, the brush pressure is reduced, so that the motor is operated in the optimum range. When the abrasion of the brush is further increased, the brush pressure is reduced further, so that the motor is operated in the electric abrasion range due to the commutation spark.
When an effective portion of the brush is worn away, the service life of the small DC motor is expired.
Accordingly, it is desirable that the brush pressure is in the optimum range shown in FIG. 1 and the variation rate of the brush pressure is small practicably through the operation of the small DC motor.
An ideal spring property for reducing the brush abrasion in consideration of the abrasion of the brush is shown in FIG. 2. Specifically, in FIG. 2, the distortion of the coil spring is small in a range of from a point O to a point A. However, this range is not normally used, but a range of from the point A to a point B is actually used in the small DC motor. Loads D and E applied to the spring correspond to the coil distortions A and B, respectively. It is desirable that the variation rate of the load applied to the coil spring between the distortions A and B is small practicably. Coil elements forming the coil spring are brought into contact with one another and the load is increased rapidly, if the distortion of the coil spring is increased from the point B to a point C. It is desirable that the range of from the point B to the point C is not used practicably. Specifically, it is preferable that a coil spring for urging the brush in the small DC motor has a non-liner spring property shown in FIG. 2.
Hitherto, a coil spring having a non-linear spring property, such as a variable pitch coil spring, a conical coil spring, a hour glass shaped coil spring, or a barrel shaped coil spring etc. has been known. However, such spring is not normally used. A coil spring having an ideal spring property shown in FIG. 2 for use in the small DC motor has not yet been obtained.
A method for obtaining a coil spring having a non-linear spring property is disclosed in the publication, “Springs” Spring Technic Research Board, published from Maruzen Kabushiki Kaisha on Dec. 20, 1982, and the publication, Toshinobu Ichiki “Theory and Practice of Brush for use in Electric Machines” published from Corona Sha on Mar. 1, 1978.
One of methods for obtaining a coil spring having a non-linear spring property is a series method wherein springs are connected in series. In this method, three coil springs having different spring constants (K1, K2 and K3), for example, are connected in series as shown in FIG. 3. The total spring constant K of the combined springs is expressed by a following formula.1/K=1/K1+1/K2+1/K3
FIG. 4 shows a relation between the distortion of the spring and the load applied to the spring having a non-linear spring property.
In the present invention, a non-linear spring device is provided by using a coil spring having a linear spring property in consideration of the method for obtaining the non-linear spring property.
FIG. 5 is a sectional view of a brush type small DC motor widely used conventionally. In FIG. 5, a reference numeral 1 denotes a motor case, and 2 denotes a permanent magnet for forming magnetic poles, provided on an inner peripheral surface of the motor case 1. A reference numeral 3 denotes a stator assembly, 4 denotes a bracket, 5 denote a brush holder for holding a brush 6 and a coil spring 7 for urging the brush 6, mounted on the bracket 4, and 8 and 9 denote motor terminals. A reference numeral 10 denotes a rotary shaft, 11 denotes an armature core, 12 denotes a winding, 13 denotes a commutator, 14 denotes a rotor assembly, and 15 and 16 denote bearings.
The brush type small DC motor is simple in construction, variable in speed, low in cost, and used widely.
FIG. 6 is a sectional view, taken along lines 6—6 of FIG. 5.